This invention relates to hydraulic jarring devices in general and more particularly to a jarring device through which impact is applied to a stuck well bore object by travel of a mandrel at an accelerated rate to an impact position.
Hydraulically controlled jarring devices for dislodging stuck objects or fish from a drilling well bore are well-known, such as the jarring devices described in my prior U.S. Pat. No. 3,729,058, and in my presently copending applications, Ser. Nos. 605,057 and 754,885.
Generally, the jarring device includes a mandrel telescopically mounted within an outer housing to form a tool adapted to be connected in a drill string through which the tool transmits an impact force to a stuck object within the well bore when an axial compression force is applied to the drill string. A fluid pressure operated valving system, located between the housing and mandrel, acts to resist any relative longitudinal movement induced by the axial compressive force. Upon release by the valving system of the fluid pressure, the mandrel is caused to move, relative to the housing, to an impact position at which a hammer surface applies a blow to an anvil surface, thereby transferring the applied force in an attempt to free the stuck object. The jarring cycle is repeated by controlling the application of the axial force at the well bore surface. It is often desirable to be able to select or vary the magnitude of the axial force, applied at the well surface, necessary to trip or complete a jarring cycle, so as to accommodate different well bore conditions. For example, in order to provide a jarring blow of adequate force, it is quite common to increase the impact mass by running a number of heavy drill collars into the well above the jarring tool. It is then desirable to select a minimum tripping force to initiate a jarring cycle and yet avoid unintentional tripping of the jarring device by the weight of the drill collars themselves. Unintentional tripping of the jarring apparatus could damage the drill bit during the drilling operation.
Very often jarring devices with limited operative capability must be used in order to apply an impact blow with increased force in order to extricate stuck objects. It is therefore desirable to provide a tool by means of which the force imparted to the mandrel may be tailored to meet certain well bore conditions.
Because of the foregoing, an important object of the present invention is to provide a jarring tool capable of applying an impact blow when tripped by a force of preselected value, the impact blow being of greater magnitude than that ordinarily produced by the initial tripping force.
It is another object of the present invention to provide a jarring tool for delivering an accelerated impact blow by internally generating a negative pressure which produces the desired acceleration.
It is yet another object of the present invention to provide a jarring tool for delivering an accelerated impact blow in the downward direction.
It is a still further object of the present invention to provide a jarring tool for delivering an accelerated impact blow in the upward direction.
The manner in which these and other objects are accomplished by the present invention will become clear from the following detailed description.